Hepatitis C virus (HCV) is the leading cause of liver disease in the United States. With no specific anti-HCV therapies, the cun-ently employed interferon-based treatment is inadequate, as it has severe side effects and is only effective in half of the major genotype infected individuals. The research proposed in this application is aimed at furthering the understanding the HCV replication mechanisms, with the ultimate goal of . . uncovering novel antiviral targets. In the past, I have studied how HCV replication is regulated, how viral proteins interact with each other and cellular factors, and helped establish systems to study HCV in cell culture. Currently, my research is focused on the earliest events in the viral life cycle involving entry into the host cell, which are poorly understood processes. In particular, although several host factors have been implicated as involved in HCV entry, litt|e is know as to how they are utilized by the virus. Our recent identification ofthe tight junction protein, claudin-1 (CLDN1), as essential for this process was a major development in this field. This discovery has opened-up a whole new view of HCV cell entry, as the involvement of CLDN1 in cell polarity strongly suggests that the polarized nature of an hepatocyte may influence how HCV enter a cell. This proposal describes experiments to;1) study HCV entry into polarized cells, 2) define how the virion interacts with and utilizes the known HCV entry factors, and 3) perform additional screens for HCV entry factors able to render both human and murine cells infectable with HCV. Such investigations will provide greater, much needed insight in HCV replication, as well as lay the foundation for future studies of replication of HCV and other related viruses.